Printers and plotters in use today for printing text/graphics typically have the capability of printing on various types of print media, such as plain paper, coated paper, and the like. Such printers and plotters typically include sensors for sensing the presence of the print medium in the media path, and the medium edges. Large scale plotters typically support roll-form print media, i.e., a supply of paper on a roll. One such device is the Design Jet product family of large scale plotters marketed by Hewlett-Packard Company. A cutter is employed to cut the medium after the plot is completed so that the finished plot is separated from the roll.
The typical printer/plotter cannot support a conventional transparent medium such as polyester. This is because a device such as the Design Jet plotter employs sensors for performing medium edge detection and medium present functions which operate by sensing reflected optical radiation from a surface of the medium. A typical transparent polyester medium such as polyethylene terephthalate (PET) has a reflectance of less than 30 percent, which is insufficient to reliably trip the optical sensor.
The use of a roll of transparent medium with a paper backing is known. The roll thus includes a layer of the transparent medium and a backing layer of paper, which is not adhered to the under surface of the transparent medium. The paper backing trips the sensors, and enables the printer to operate on the transparent medium. This technique suffers from several disadvantages. The roll trailing edges do not necessarily line up or match, causing a loss of expensive print media at the roll end when the paper backing ends before the transparent medium. Further, the paper thickness adds to the weight of the roll, reduces the length of print medium which can be put on a roll of a given thickness, and increases the media advancement error. A further disadvantage is that, due to hygroscopic and thermal expansion effects of the paper backing, the roll can telescope lengthwise, which in extreme cases could prevent the roll from being loaded into the machine. The lateral edges of the paper backing may not coincide with the edges of the transparent medium, leading to edge sensing inaccuracies.